One of the many functional requirements of power supplies is to generate regulated sinusoidal power for various sensor excitations. In producing the sinusoidal output, maximum efficiency is desirable for minimum heat generation, maximum reliability, and minimum system weight.
Present day sinusoid generator designs utilize complimentary power MOSFET devices in the output stage operating in a symmetrical "Class AB" mode. The theoretical maximum efficiency obtainable in this mode of operation is limited to only 78%. This number may be derived by classical load-line analysis and is based on ideal circuit components having zero source impedance, infinite input impedance and infinite gain. For actual circuits, however, ideal parameters do not exist and the maximum obtainable efficiency is, therefore, less than 78%.
The "Class D" mode of operation, which utilizes the basic principle of pulse width modulation to generate a sine wave by filtering a frequency modulated square wave, allows theoretical efficiencies approaching 100%. However, implementation of this design using bipolar transistors has not generally been successful due to the inherent limitations of transistor switching times and drive requirements.